1. Field of Disclosure
Embodiments of the inventive concepts described herein relate to a positive electrode active material for lithium secondary battery, and more particularly, relate to a positive electrode active material for lithium secondary battery which includes a first concentration gradient portion, a second concentration gradient portion, and a first concentration maintained portion. The first and second concentration gradient portions have gradients of concentrations of nickel, manganese, and cobalt in the direction from the center to the surface, and the first concentration maintained portion has constant concentrations of nickel, manganese, and cobalt between the first concentration gradient portion and the second concentration gradient portion.
2. Description of the Related Art
A lithium secondary battery has an operating voltage of 3.7 V or more, and the demand for the lithium secondary battery as a power source for driving portable electronic information and communication devices is increasing day by day since the energy density per unit weight thereof is higher than a nickel-cadmium battery or a nickel-hydrogen battery.
In recent years, the study to hybridize an internal combustion engine and a lithium secondary battery as a power source for an electric vehicle has been actively carried out in US, Japan, Europe, and the like. A battery for plug-in hybrid (PHEV) used in a vehicle which has a travel distance of less than 60 miles a day is actively developed mainly in US. The battery for PHEV is a battery which has characteristics close to an electric vehicle, and thus it is a great task to develop a battery having a high capacity. In particular, it is the most important task to develop a positive electrode material which has a high tap density of 2.0 g/cc or more and a high capacity of 230 mAh/g or more.
The positive electrode material which has been currently commercialized or is being developed includes LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li1+x[Mn2−xMx]O4, LiFePO4, and the like. Among these, LiCoO2 is an excellent material which has stable charge and discharge characteristics, excellent electron conductivity, a high battery voltage, high stability, and flat discharge voltage characteristics. However, Co is expensive and toxic to a human body and its reserves is little, and thus the development of another positive electrode material is desired. Moreover, the thermal characteristics of LiCoO2 are significantly poor since its crystal structure is unstable due to delithiation during charge.
To improve this, it is attempted that the start temperature of heat generation is shifted to a higher temperature side by substituting some of Ni with transition metal elements or the exothermic peak is made broad so as to prevent a rapid heat generation. However, satisfactory results have not yet been obtained. In other words, the material LiNi1−xCoxO2 (x=0.1-0.3) obtained by substituting some of Ni with Co exhibits excellent charge and discharge characteristics and lifespan characteristics but the problem of thermal stability has not yet been solved. In addition, in European Patent No. 0872450, the material LiaCobMncMdNi1−(b+c+d)O2 (M=B, Al, Si. Fe, Cr, Cu, Zn, W, Ti, Ga) obtained by substituting some of Ni with other metals as well as Co and Mn is disclosed, but the thermal stability of the Ni-based material has not yet been solved.
To overcome such a disadvantage, a lithium transition metal oxide which has a concentration gradient of the metal composition has been proposed in Korea Patent Publication No. 2005-0083869. In this method, a material is produced in a double layer by once synthesizing an inner material having a certain composition and then covering it with a material having a different composition and then mixed with a lithium salt, and the mixture is subjected to the heat treatment. A commercially available lithium transition metal oxide may be used as the inner material.
In this method, however, the metal composition of the positive electrode active material discontinuously changes between the inner material and the outer material thus produced but not continuously and gradually changes. Moreover, the powder synthesized by this invention has a low tap density since ammonia of a chelating agent is not used therein, and thus it is not suitable for being used as a positive electrode active material for a lithium secondary battery.
In order to improve this point, in Korea Patent Publication No. 2007-0097923, a positive electrode active material which has an inner bulk portion and an outer bulk portion and a continuous concentration distribution of the metal components depending on the location in the outer bulk portion has been proposed. However, in this method, the concentration is constant in the inner bulk portion and the metal composition is only changed in the outer bulk portion, and thus it is desired to develop a positive electrode active material having a new structure so as to exhibit excellent stability and to have a higher capacity.
In addition, the reversible capacity relatively increases but the thermal stability significantly decreases when the content of Ni is high, and when the content of Ni is relatively low and the content of Mn is high, the thermal stability is improved but the advantage in terms of energy density is eliminated as compared to the existing LiCoO2. Hence, it is desired to have the optimum composition of Ni:Mn:Co and the ratio of Li/M in terms of capacity and safety in order to completely or partly replace the existing LiCoO2.
The control of the ratio of Li/M in the positive electrode active material is associated with the content of Mn in the transition metal composite, and it is possible to insert an excess amount of lithium into the transition metal layer when the amount of Mn substituted is equal to or more than a certain value. In terms of battery characteristics, a relatively high high-rate characteristics and lifespan characteristics are exerted by the excess amount of lithium inserted into the transition metal layer, and it is easy to insert lithium into the transition metal layer in a composition system having a relatively high content of Mn as compared to the ternary system having a low content of Mn, thus it is possible to control the content of water-soluble bases such as Li2CO3 and LiOH which remain on the surface of the active material after firing by minimizing the amount of lithium added at the time of synthesis. The residual lithium components are decomposed during charge and discharge or react with the electrolytic solution to generate CO2 gas, as a result, the swelling phenomenon of the battery is caused and particularly the high-temperature stability of the battery deteriorates.
In particular, impurities such as LiOH or Li2CO3 are formed on the surface when a ternary positive electrode active material containing Ni as a main component is exposed to air and moisture (see Reaction Formulas 1 and 2; J. Power Sources, 134, page 293, 2004)LiNiO2+yH2O→Li1−yNiO2−y/2+yLiOH  Reaction Formula 1:LiNi0.8Co0.15Al0.05O2+4×O2+yH2O→Li1−yNi0.8Co0.15Al0.05O2+2×Li2Co3  Reaction Formula 2:
The residual lithium component thus formed raise the pH during the preparation of slurry for plate, the slurry containing NMP (1-methyl-2-pyrrolidinone) and a binder begins to be polymerized to form gel, and thus a problem is caused in the plate producing process. Lithium hydroxide decreases the dispersibility of the positive electrode active material, a binder, a conductive material, and the like in a solvent, and thus it takes a long period of time to stabilize the viscosity of the slurry. In addition, when the slurry is coated on the current collector while the viscosity is not stabilized, it is not uniformly coated on the current collector, the smoothness of the electrode surface decreases, and thus the performance of the battery deteriorates.
Hence, a number of prior arts have focused on improving the characteristics and the producing process of the positive electrode active material containing nickel as a main component in order to decrease the amount of residual lithium.